1. Technical Field
The present invention relates to a quadrature voltage controlled oscillator (VCO), and more particularly, to a quadrature VCO capable of varying a phase difference between an in-phase output signal and a quadrature output signal.
2. Discussion of the Related Art
Current mobile communication systems use low-intermediate frequency (Low-IF) way or direct-conversion way rather than heterodyne way to convert a radio frequency (RF) signal to a baseband signal. Thus, a VCO used in such mobile communication systems should generate both an in-phase output signal (I output signal) and a quadrature output signal (Q output signal).
The I and Q output signals may be generated using a poly-phase-filter shown in FIG. 1, a divider shown in FIG. 2, or two VCOs shown in FIG. 3.
When the I and Q output signals are generated using the poly-phase-filter as shown in FIG. 1, the poly-phase-filter is located in a path through which signals pass. Thus, signal loss occurs, making it difficult to generate I and Q output signals. In addition, the I and Q output signals are affected by the operation frequency and layout of the poly-phase-filter. When the I and Q output signals are generated using the divider as shown in FIG. 2, precise I and Q output signals can be obtained. However, a VCO, which oscillates at a frequency double a desired frequency, is required. When the I and Q output signals are generated using two VCOs as shown in FIG. 3, the two VCOs have the same structure and form a feedback loop therebetween. As a result, the I and Q output signals are spontaneously generated. This is disclosed in U.S. Pat. No. 6,462,626, issued on Oct. 8, 2002, entitled “Quadrature Output Oscillator Device.”
The above-described methods have a common problem in that a phase difference between the I and Q output signals is ideally fixed at 90° or substantially fixed at 90°±offset. In other words, when the I and Q output signals are applied from a VCO to a down-mixer in the direct-conversion method, the down-converted signals should be ideally 90° out of phase, but substantially not. This results in deterioration of a bit error rate of a transceiver.
In addition, in the Low-IF method, the image-rejection characteristics of an RF signal should be good. However, it is difficult to expect good image-rejection characteristics from the above-described methods.